For vessels exposed to repeated internal pressure cycles, annular cracks typically grow outward from some type of discontinuity such as a blind end closure, threaded end closure, etc. The cracks continue to propagate toward the outer surface of the vessel. Failure occurs when the remaining intact material can no longer withstand the pressure load being exerted on the area exposed by the crack. The eventual failure (burst) may be violent since the stored energy of the system is quickly released and in some cases material is ejected from the failure point. In general, designs in which annular fatigue cracks grow due to pressure cycling should be avoided. However, on occasion designs of this type are necessary. FIG. 1 is a cross-sectional view illustrating a pressure member 102 with a typical annular crack 104 originating at the radius of a blind end closure within the pressure member, as may have been experienced according to the prior art.
One reason burst type failures can be hazardous is related to the possible ejection of material. FIG. 2 is a perspective view of a three pound piece of material 202 that was ejected during such a failure. The stored energy within the system when the pressure member failed was approximately 1,000,000 ft-lbs. The failed surface of the piece 202 illustrates classic annular crack growth 204 corresponding to fatigue, and eventual failure. The crack initiated at a discontinuity 206 created at the intersection of two bores.